The invention relates to secondary suspensions for rail vehicles, and more particularly to a secondary suspension for a rail vehicle whose mounting position, or mounting height, can be altered by means of supplementary elements.
Low-level rail vehicles of conventional design have a free space located between the running gear and the car body. This free space is generally determined by two principal factors. First, a travel distance of secondary spring deflections must be considered. This factor is dynamic and depends on a load carried.
The second factor includes the geometrical boundary conditions resulting from travel on a summit, an incline, or upon a ramp. These latter conditions may be temporarily disregarded for a stationary vehicle.
The long standing problem of reducing the free space, between the running gear and the car body, may be addressed by supplementing the secondary suspension of a vehicle. The floor level, and thus the level of the boarding ledge, may be reduced by decreasing the height of this free space. Numerous attempts have been made to solve issues related to this problem in the devices of the prior art.
A car jacking device is disclosed in U.S. Pat. No. 2,917,321 which shows a static shock absorber. The simply constructed shock absorber includes a locking device to prevent movement of the chassis of the vehicle relative to a suspension system thereof. However, no mechanism to dynamically adjust for changing geometrical boundary conditions is disclosed.
U.S. Pat. No. 3,524,657 discloses a composite spring assembly for a vehicle. This composite spring assembly uses a constant rate spring in parallel with a vacuum actuated variable rate spring. Constraints related to the size of the vacuum spring assembly, however, necessitate a remote location of the coil spring.
U.S. Pat. No. 4,097,068 discloses a pneumatic-coil spring suspensions which has a pneumatic spring component located below a coil spring component to substantially inhibit tilting of the pneumatic spring component. This suspension, however, employs a fixed spring seat for the pneumatic spring component.
In order to selectively allow a car body to be lowered, (for example, at train stops) a device is required for variably shortening the suspension system of a rail vehicle without compromising the dynamic operation of the suspension system during travel. Thus, elements must be provided which function as a damper during travel, as well as functioning to shorten the secondary spring mechanism while stationary.